The present invention relates generally to vehicle hydraulic systems, and more particularly, to such systems including two or more hydraulic actuators which, under certain conditions, must be given priority over other vehicle hydraulic functions.
Although the present invention may be used to control the flow of fluid to many different types of fluid actuated systems and components, it is especially useful in a vehicle hydraulic system including some sort of hydrostatic steering control system and a hydraulic power brake system, and will be described in connection therewith.
Off-highway vehicles, such as agricultural tractors and construction equipment, using both hydrostatic steering systems and hydraulic brake systems have been known for many years. For safety reasons, there are times when the hydrostatic steering system needs priority over all other vehicle hydraulic functions. On the other hand, there are times when, also for safety reasons, the hydraulic brake system must have priority over all other hydraulic functions on the vehicle.
One of the common approaches to the situation wherein either the steering system or brake system may require priority has been to provide two pumps to supply the system. Normally, in such an arrangement, one of the pumps feeds the steering system while the other pump feeds the brake system, and excess fluid from the pumps, not required by either steering or brakes, may be utilized by one or more auxiliary load circuits. It has been recognized that a system including two pumps will inherently result in a greater waste of pump input horsepower, especially when neither the steering nor the brake system is being utilized.
Partly in response to the need to conserve pump input horsepower on such vehicles, the trend in mobile hydraulics has been toward greater use of "load sensing" hydraulic components, i.e., those which sense the load or pressure on the associated fluid actuated device and transmit a load signal to the variable source of fluid which, in response to the load signal, adjusts its fluid output to satisfy the demand for fluid. The trend toward the use of load sensing on vehicle hydraulic components has probably been greatest in the case of hydrostatic steering systems.